This year, the CBS Battleground Tracker “is concentrating on the districts that we think will make a difference,” he said. “Remember, in a midterm, you have to watch each congressional seat — don’t pay attention to national numbers.”

Just as a presidential race is not a national contest but a collection of 51 separate elections (one for each state and for the District of Columbia), November’s midterm involves elections for 435 House districts and 35 Senate seats. […]